Flat panel displays, such as liquid crystal displays (“LCDs”) and organic light emitting displays (“OLEDs”), typically have a matrix of display pixel elements arranged in rows and columns that is driven by a display driver circuit. The display driver circuit includes driver units that provide column line outputs to drive display pixel elements in respective columns of the display matrix. The display image quality depends on uniformity of the column line outputs provided by the driver units of the display driver circuit. When there are driver non-uniformities among the column line outputs, the output signals supplied to the column lines may not accurately drive the display pixel elements according to input display data signals. More specifically, when there are non-uniformities among the column line outputs, the pixel brightness of each pixel element may not conform to the desired brightness. For color images, such non-conformance can lead to color non-uniformities. Thus, the display image quality depends on the drive uniformity of the column line outputs.
Such non-uniformities in pixel brightness and color may be found in all flat-panel display systems, including both passive matrix and active matrix types of display systems. Manufacturing variations result in parameter variations in the integrated circuitry of the display driver circuit, leading to performance mismatches between otherwise identically designed circuits. In response, circuit designers can make use of device dimensions, such as area, width, and length; device layout; circuit configuration; and device bias point to control mismatching. Nevertheless, in display driver circuits, both systematic and random variations may occur in large numbers of identically designed drive units for respective columns of the display matrix and affect the quality of the displayed image. Thus, because there are typically hundreds of driver units in a display driver circuit, there is greater potential for drive non-uniformity due to manufacturing variations in the integrated circuit.
Using an active matrix thin film transistor (“TFT”) liquid crystal display as an example, FIG. 1 shows a gamma curve that indicates a relationship of output brightness level of the display to a driver unit output voltage level to the column lines. The gamma curve generally corresponds at least to all pixel elements of the same color, and can correspond to all pixel elements. The output voltage of each driver unit can take on different voltage levels, e.g., VGL0 to VGL63, respectively corresponding to grey levels of display brightness, e.g., GL0 to GL63 in the case of six-bit display data. For any row of display pixel elements, when the display data are the same across several columns, the outputs of the driver units for those columns should be the same voltage level in order to drive the adjacent display pixels elements at the same brightness. In practice, however, because of manufacturing variations, the output levels among the identically designed driver units may exhibit small variations. As seen in Case 1 in FIG. 1, two identical driver units produce Output 1 and Output 2 for adjacent pixel elements pixel 1 and pixel 2, that vary from each other and from the desired grey level of VGL(N). Ideally, for uniform drive, the two identical driver units should produce the same output, i.e., Output 1=Output 2, as seen in Case 2 in FIG. 1.
Visual perception is more sensitive to the effect of small output variations in close proximity than to the effect of small offsets from an ideal absolute output level. Thus, output variations of adjacent driver units are more visually noticeable. Several approaches have been taken to reduce output non-uniformity. First, in designing driver units, the direct approach to achieve output uniformity is to reduce the design's sensitivity to process variations. This approach uses large device dimensions, such as area, width, length, and spacing to minimize the effects of manufacturing variations. An example of this approach can be found in Kinget, Peter R., “Device Mismatch and Tradeoffs in the Design of Analog Circuits,” IEEE J. Solid-State Circuits, vol. 40, no. 6, pp. 1212-24, June 2005.
Another approach to increase output drive uniformity uses physical layout techniques of symmetry and common-centroid to average the effects of manufacturing variations. Such methods can reduce the offsets and variations in the outputs of driver units. Buffer amplifier offset can also be a significant cause of driver unit output non-uniformity. Examples of ways to reduce buffer amplifier offset are, for example, by autocharge-compensated sampling, such as described in Shima, T. et al., “Principle and Applications of an Autocharge-compensated Sample and Hold Circuit,” IEEE J. Solid-State Circuits, vol. 30, no. 8, pp. 906-12, August 2005, or by switch capacitor offset compensation techniques, such as described in Bell, Marshall, “An LCD Column Driver Using a Switch Capacitor DAC,” IEEE J. Solid-State Circuits, vol. 40, no. 12, pp. 2756-65, December 2005.
Yet another technique for increasing drive uniformity is a “multi-driving” approach described in Korean patent number KR2003056005 and shown in FIG. 2. This multi-driving circuit includes a resistive voltage divider 20, a first amplifier 21, and a second amplifier group, comprised of amplifiers 22a, 22b, and 22c. Voltage divider 20 divides a predetermined gamma reference voltage and outputs the divided voltages V(m) to first amplifiers, such as first amplifier 21. First amplifier 21 amplifies the divided voltage and sends the divided voltage to a set of decoders 26. Amplifiers 22a, 22b, and 22c, in the second amplifier group, receive respective output signals from a set of decoders 26 and provide power to drive a load at the output to the predetermined gamma reference voltage. During operation, column line outputs, Y1, Y2, and Y3, are respectively driven by amplifiers 22a, 22b, and 22c. Amplifiers 22a, 22b, and 22c of the second amplifier group have high slew-rate properties, which provides a fast response for the column line outputs Y1, Y2, and Y3. Near the end of the line period, column line outputs Y1, Y2, and Y3 are coupled to decoder outputs by switches 25a, 25b, and 25c. Output non-uniformities due to driving variations of amplifiers 22a, 22b, and 22c are thus averaged, since the outputs of individual decoders 26a, 26b, and 26c of the set of decoders 26, are all driven from first amplifier 21.
This multi-driving approach requires first amplifier 21 to have large driving capability for driving column line outputs Y1, Y2, and Y3, and to maintain stability under a range of loading conditions that are dependent on display data. In the traditional voltage divider approach, first amplifiers 21 are deployed infrequently at relatively few divider points. This multi-driving approach may require first amplifiers 21 at more divider points to reduce the loading effects on resistive voltage divider 20. Timing control of switches 25a, 25b, and 25c is also required for the operation and can become more difficult with increasing display resolution and display size.
The techniques mentioned above can improve drive uniformity, but they require significant additional circuitries, silicon area, and/or power consumption to minimize drive non-uniformities. Even with the techniques described above, some small non-uniformities will invariably remain due to practical limits, such as the acceptable amount of increase to device dimensions or layout configurations. For example, autocharge-compensated sampling and switch capacitor offset compensation may be undesirable choices for driver unit design because, due to the large number of outputs requiring compensation, they may require unacceptably large amounts of additional silicon area and may consume large amounts of power. In addition, switch capacitor techniques may require special attention to issues of charge injection, nonlinear MOS capacitor characteristics, switch size effects, critical timing of control signals, and unavoidable non-uniformities due to process variations. Thus, some level of driver unit output non-uniformity will remain due to practical limits in resolving such issues.